This invention relates to roller swaging of tubing, such as for attaching hydraulic fitting sleeves to high strength, thin wall, aerospace hydraulic tubing. Such swaging machines are disclosed in U.S. Pat. No. 4,658,616 (Bastone) assigned to the assignee of this application. The contents of that patent are incorporated by reference herein.
Roller swaging of tubing for attachment of hydraulic fittings is a common practice in the aerospace industry. In roller swaging, the end of an expander assembly is inserted into the tube to be swaged. At the time of insertion, a sleeve to be swaged onto the tube is loosely in place on the tube. The end of the expander assembly includes a plurality of rollers that are free to move radially toward and away from the longitudinal axis of the expander assembly. A rotating tapered mandrel is moved along the axis of the expander assembly, frictionally engaging the rollers and forcing the rollers against the inner wall of the tube. The mandrel continues to rotate and advance, causing the rollers to expand the tube, forcing tube material to flow into grooves in the sleeve, effecting a strong sealed connection between the tube and the sleeve.
Prior art roller swaging have exhibited some disadvantages. These include poor control being either insufficient or too much swaging. Either condition can result in leakage, which is a particular problem with titanium tubing.
There is also low speed operation. Manual adjustments and operations are time consuming and therefore expensive. When titanium tubing is used, a slow rate of swaging is required to prevent the tubing from springing back to a smaller diameter following completion of the swage, which can result in failure of the joint. With the slow rate, however, the cycle time can be too long for efficient operation of the swaging machine.
Generally, roller swaging in the prior art employs either a torque control of the swaging tool whereby the torque measures the amount of swage. Alternatively, a diameter control is used wherein the inside diameter measurement of the swaged tube is used to indicate the degree of swage. This diameter measurement control is effected mechanically and is prone to error due to error in the mechanical settings.
In the method of diameter control it has been found that when swages are made to a selected inner tube diameter, the strength of the swaged connection is not as consistent as desired. This lack of consistency is due to variations in tube wall thickness and diameter, and to a lesser extent, sleeve wall thickness and diameter. Moreover, it is relatively difficult to effect accurately the correct diameter settings. Diameter control is however advantageous in that the expander units are less costly since they do not have to be calibrated and certified to swage at a specific torque range as do expanders working in the torque control mode.
Roller swaging under torque control exhibit one or more of the following disadvantages:
1. Individual, interchangeable, pre-set torque limiters are awkward to exchange and store, and impractical to adjust;
2. Torque sensing by measurement of electrical current in a mandrel drive motor can be inaccurate;
3. Torque limiting based on balancing a gimballed motor housing against an adjustable bias such as a spring requires time-consuming manual adjustment of the bias to effect a change in the limiting torque;
4. Torque limiting by manual use of a torque wrench driving the mandrel is too slow for economical production and subject to operator-induced variation; and
5. Non-uniform swaging results from failure to compensate for the torque required to turn the mandrel before swaging actually begins.
A torque-controlled swaging machine of U.S. Pat. No. 4,658,616 (Bastone) discloses a technique for high speed-low speed swaging under control of a torque transducer which monitors torque. This apparatus however does not optimise the swaging effect and also does not take into account the positive effects of diameter control of swaging. Moreover, with larger diameter tubing and a low-speed which is relatively high, namely, about 50% of the high speed, the final torque is relatively difficult to control. Accordingly, the final swage level also proves difficult to maintain accurately.
It is also generally desirable to finish a swaged joint with a burnishing effect. This is not effected with the torque control machines of the prior art. Mechanical diameter control swaging machines have used burnishing, however, the disadvantage of that procedure is that the tool life is shortened due to the interaction of the swaging rollers on the mandrel which the swaging forces are extensive and the speed of rotation is high.
Thus there is a need for a roller swaging machine which can have the benefits of both torque control and diameter control, burnishing, close control of the amount of swaging, has long tool life, swages at an overall high cycle speed, is easy to use, and is automatic in operation.